Experimental Study On Amplitudefrequency Characteristics Of Monopole Inner And Outer Collar Waves

Acoustic logging while drilling(LWD)is a widely used logging technology in the field of oil and gas exploration,and it has rapidly became a mainstream technology due to its distinguished advantages.However,for this technique there is still one shortcoming due to the special structure of the logging probe.Specifically,the highamplitude collar waves can bring troubles in picking up formation information.On this issue,scholars have conducted a lot of theoretical and experimental research,but this difficulty has not been completely overcame yet.As the research progresses,it is found that the drill collar wave propagates inside the drill collar but also radiates energy to its outside.Therefore,in this paper,we focus on the amplitude-frequency response of the inner and outer collar waves,and summarize a method to extract formation longitudinal velocity.This method is based on elimination of the interference in the full waveforms by using the inner collar waves.Our method obtains satisfactory experimental results in the longitudinal velocity measurement of both soft and hard formation models.Firstly,we show the details of the hardware for the small-size model of the LWD experiment.These informations include the equipment type,equipment performance,material properties of transducers and model size.Subsequently,the material properties,dimensional parameters and precautions for the LWD probe and transducer are also explained in this paper.Then,the specific experimental procedures and operation methods are described.For the problems of electromagnetic interference between the electronic equipment in the laboratory,the corresponding means of shielding and reducing electromagnetic interference are given in this paper.Secondly,we conduct an experimental study on the measurement of inner and outer collar waves in the fluid model,the results show that both the inner and outer collar waves can be efficiently measured and collected separately.On this basis,the amplitude decay characteristics of the two collar waves along the drill collar axis are analyzed and studied separately.The results based on the exponential function show that the outer collar wave decays faster than the inner collar wave,but since the average amplitude of the inner collar wave is 3.4 times that of the outer collar wave,they are in the same order of magnitude,so the outer collar wave should not be ignored when studying the borehole sound field.Next,the amplitude-frequency response of the inner and outer collar waves is studied in this paper.While keeping the source voltage constantly,the source frequency is varied,and the amplitude changes of the inner and outer collar waves are subsequently analyzed.The experimental results indicate that the center frequencies of the inner and outer collar waves are 100 k Hz and 80 k Hz,respectively,and the change rates of both rising and falling of the inner collar waves are faster than those of the outer collar waves.After comparing the amplitude-frequency response of the formation longitudinal and transverse waves,we find that the central frequency bands of the collar waves and the formation waves overlap seriously.Although increasing the sound source frequency can reduce the amplitude ratio of the collar waves and the formation waves,the amplitude ratio of the inner collar wave and the formation longitudinal wave,for example,is still as high as about 25 at the highest frequency(400 k Hz)of the sound source in the experiment.Therefore,adjusting the sound source frequency has limited effect on improving the signal-to-noise ratio of the formation waves in the LWD signals.In addition,the spectral analysis of the wire-line logging signals and the inner collar wave shows that the main band of the stratigraphic longitudinal and transverse waves and the inner collar wave also overlap seriously,so that the traditional signal filtering means cannot filter out the collar wave cleanly.Finally,by comparing the time domain characteristics of the inner and outer collar waves and the full waveforms,and taking the distribution characteristics of the LWD signals into account,we initially propose an idea of using the internal and outer collar waves to eliminate the interference of the collar waves in the full waveforms.After the analysis and research,the idea is further summarized in this paper,that is,using the processed inner collar wave to remove the collar waves from the full waveforms can effectively extract the formation longitudinal wave signal,and then realize the accurate measurement of the formation longitudinal wave velocity.The experimental results in sandstone and nylon model wells show that the stratigraphic longitudinal wave extraction by this method is effective and the wave velocity measurement is accurate,which further verifies the feasibility and effectiveness of this method.